The invention relates to methods for the electrical stimulation of the auditory nerve, and to multichannel hearing prosthesis for carrying out the method. Such methods and prostheses are e.g. known from IEEE Journal of Solid-State Circuits, Vol. SC-10, No. 6, December 1975, pages 472 through 479.
The advancing miniaturization of electronic circuits has led to the manufacture of small stimulation current transmitters which can be implanted in the body for the electric stimulation of nerves and muscles. In addition to serving the function of the stimulation of the heart muscles (heart pacemakers), etc., circuits have also become known which are suitable for stimulation of the auditory nerves. The single channel and multichannel electrode systems are so constructed that they can deliver small stimulation currents. However, they can only be employed for their function as a hearing prosthesis for deaf individuals if the inner ear is, indeed, non-functional, but if the auditory nerve, including higher processing locations for information transmission and information processing, is still intact. In the case of deaf individuals with such impairments, a small receiver can be implanted in the mastoid. Stimulation currents can then be transmitted from the implant via an electrode bundle. The signals are generated, in a portion of the apparatus worn outside the body, from the sound events which are to be conveyed to the wearer of the apparatus, and are transmitted wirelessly via a small transmitter to the implanted receiver in order to avoid an electrically conductive connection through the skin and the related risks of an infection.
The invention proceeds from the assumption that there are per se no significant restrictions regarding construction and size for the portion of the apparatus worn outside the body, i.e., the converter of the sound events into transmittable signals, and for the transmitter, whereas, for the implanted portion, one must proceed from specific constraints such as, for example:
1. The receiver is to possess a small volume (maximally 2 cm.sup.3), so that the electrodes can remain short. Long electrode wires result in electric (cross-talk) and mechanical (wire breakage during movement) problems.
2. At least ten to twenty electrodes (according to the application up to twenty-four) are to be capable of being provided, which, in the range of frequencies of between 100 and 5000 Hz, can deliver stimulation currents of at least ten microampers (10 .mu.A), whereby the form of the signals is to be freely selectable within wide boundaries, in order that, following completed implantation, the optimum stimulation current form can be found and adjusted with the patient in experiments.
The internal resistance of the circuit should be as high as possible in order that the current is impressed at the transmission points of the electrodes (electrode tips). However, simultaneously, with regard to electrolysis, the voltage cannot be permitted to become excessively high in order to avoid a damage to the surrounding tissue. If necessary, a current source with voltage limitation would have to be provided for this purpose.
3. The separation of the present channels is to amount to at least thirty decibels (30 dB); i.e., if a stimulation current J.sub.i is generated at an electrode, in the interest of high channel separation, the current brought about by J.sub.i at another electrode should be less than J.sub.i /32. The fraction with the denominator thirty-two results from the 30 dB. A value which is greater would further improve the channel separation; i.e., reduce the cross-talk; a value which is smaller would reduce the channel separation to low values.
Moreover, the implanted materials must be compatible with the tissue. The materials cannot be permitted to change even after years of implantation.